Microstrip filter

ABSTRACT

There is disclosed herein a microstrip filter having low loss and high rejection at X-band. The filter includes N microstrip open circuited, half-wavelength hairpin resonators disposed in a series coupled relationship with respect to each other, where N is an integer equal to or greater than one. A metallic housing encloses the hairpin resonators for shielding thereof to increase the unloaded Q of the resonators. An improved signal input and output coupling arrangement is also provided for the filter. The signal coupling arrangements include a microstrip conductor having a first width less than the width of the microstrip conductor forming part of each of the hairpin resonators between the input and output hairpin resonators. In the coupling arrangement, the adjacent limb and a portion of the cross-arm connected to the adjacent limb of the conductor of the input and output hairpin resonators also have said first width and the spacing of the limbs of the conductors of the input and output hairpin resonators are greater than the spacing of the limbs of the conductors of the hairpin resonators between the input and output hairpin resonators.

I United States Patent 1 Anghel Aug. 21, 1973 MICROSTRIP FILTER SeverAnghel, Milltown, NJ.

[73] Assignee: International Telephone and Telegraph Corporation,Nutley, NJ.

22 Filed: Mar. 20, 1972 211 Appl. No.: 236,283

[75] Inventor:

Primary Examiner-Rud0lph V. Rolinec Assistant Examiner-Hugh D. JaegerAttorney-C. Cornell Remsen, Jr., Menotti J. Lombardi, Jr. et al.

[57] ABSTRACT There is disclosed herein a microstrip filter having lowloss and high rejection at X-band. The filter includes N microstrip opencircuited, half-wavelength hairpin resonators disposed in a seriescoupled relationship with respect to each other, where N is an integerequal to or greater than one. A metallic housing encloses the hairpinresonators for shielding thereof to increase the unloaded Q of theresonators. An improved signal input and output coupling arrangement isalso provided for the filter. The signal coupling arrangements include amicrostrip conductor having a first width less than the width of themicrostrip conductor forming part of each of the hairpin resonatorsbetween the input and output hairpin resonators. In the couplingarrangement, the adjacent limb and a portion of the cross-arm connectedto the adjacent limb of the conductor of the input and output hairpinresonators also have said first width and the spacing of the limbs ofthe conductors of the input and output hairpin resonators are greaterthan the spacing of the limbs of the conductors of the hairpinresonators between the input and output hairpin resonators.

10 Claims, 6 Drawing Figures MICROSTRIP FILTER BACKGROUND OF THEINVENTION This invention relates to microstrip filters and moreparticularly to microstrip filters operable at X-band.

Microstrip filters for X-band operation are generally lossy, and alsoprovide very limited rejection out-ofband. The problem is attributed toradiation into the free space above the microstrip substrate. Thefollowing articles have suggested techniques to overcome this problem.(1) R. J. Roberts and B. Easter, Microstrip Resonators Having ReducedRadiation Loss," Electronic Letters, Vol. 7, No. 8, pp. 191-192, Apr.1971; (2) A. F. Hinte, G. Kopcsay and J. J. Taub, Alumina Sandwich LineFilters for High Performance Integrated Circuit Applications, 1971 G-MTTSymposium Digest, pp. 26-27; and (3) A. R. Chinchillo and R. W. Perry,Microstrip Filter Loss Reduction Techniques, NEREM 1970 Record, pp.72-73.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a microstrip filter for operation at X-band having low loss andhigh rejection employing a technique different than techniques describedin the above-identified prior art articles.

A further object of the present invention is to provide a microstripfilter for X-band operation employing microstrip open circuited,half-wavelength hairpin resonators enclosed in a non-critical enclosurefor shielding of the resonators.

Another object of the present invention is to provide a microstripfilter for X-band operation employing microstrip open circuited,half-wavelength hairpin resonators enclosed in a non-critical enclosurefor shielding of the resonators having an improved arrangement to couplesignals to and to couple signals from the microstrip filter.

A feature of the present invention is the provision of a microstripfilter comprising: N microstrip open circuited, half-wavelength hairpinresonators including a ground plane, a substrate disposed adjacent to,coextensive with and parallel to the ground plane and N hairpin shapedconductors disposed in series coupled relationship with each other onone surface of the substrate opposite the ground plane, where N is aninteger equal to or greater than one, each of the N conductors includinga pair of parallel spaced limbs interconnected by a cross-arm, adjacentones of the N conductors having an opposite orientation with respect toeach other and adjacent ones of the limbs of adjacent ones of the Nconductors being in spaced parallel relationship with respect to eachother to provide the series coupled relationship; a microstrip inputcoupler including a first conductor disposed on the one surface of thesubstrate in spaced, series coupled relationship with an adjacent one ofthe limbs of a first of the N conductors; a microstrip output couplerincluding a second conductor disposed on the one surface of thesubstrate in spaced series coupled relationship with an adjacent one ofthe limbs of a last of the N conductor; and metallic members coextensivewith the ground plane and the substrate enclosing the one surface of thesubstrate and the first, second and N conductors to shield the hairpinresonators to increase the unloaded Q of the resonators with respect tothe unloaded Q obtainable with unshielded hairpin resonators.

Still another feature of the present invention is the provision of amicrostrip filter comprising: N microstrip open circuited,half-wavelength hairpin resonators including a ground plane, a substratedisposed adjacent to, coextensive with and parallel to the ground planeand N hairpin shaped conductors disposed in series coupled relationshipwith each other on one surface of the substrateopposite the groundplane, where N is equal to an integer greater than two, each of the Nconductors including a pair of parallel spaced limbs interconnected by across-arm, adjacent ones of the N conductors having an oppositeorientation with respect to each other and adjacent ones of the limbs ofadjacent ones of the N conductors being in spaced parallel relationshipwith respect to each other to provide the series coupled relationship; amicrostrip input coupler including a first conductor disposed on the onesurface of the substrate in spaced, series coupled relationship with anadjacent one of the limbs of a first of the N conductors; and amicrostrip output coupler including a second conductor disposed on theone surface of the substrate in spaced, series coupled relationship withan adjacent one of the limbs of a last of said N conductors; each of thelimbs and the cross-arm of each of the N conductors between the firstand last of the N conductors having a width equal to a first givenamount; the spacing between each of the limbs of each of the Nconductors between the first and last of the N conductors is equal tothe first given amount; the first conductor including a first portionparallel to the adjacent one of the limbs of the first of the Nconductors and a second portion perpendicular to the first portion, thefirst portion and a section of the second portion directly connected tothe first portion each having a width equal to a second given amountless than the first given amount; the second conductor including a thirdportion parallel to the adjacent one of the limbs of the last of the Nconductors and a fourth portion perpendicular to the third portion, thethird portion and a section of the fourth portion directly connected tothe third portion each having a width equal to the second given amount;and the adjacent one of the limbs of each of the first and last of the Nconductors and a section of the cross-arms directly connected to theadjacent one of the limbs of each of the first and last of the Nconductors having a width equal to the second given amount, the other ofthe limbs of each of the first and last of the N conductors and theremainder of the cross-arms connected to the other of the limbs of eachof the first and last of the N conductors having a width equal to thefirst given amount and the spacing between the adjacent one and theother of the limbs of each of the first and last of the N conductors isgreater than the first given amount.

BRIEF DESCRIPTION OF THE DRAWING Above-mentioned and other features andobjects of this invention will become more apparent by reference to thefollowing description taken in conjunction with the accompanyingdrawing, in which:

FIG. 1 is a perspective view with the cover removed of a microstripfilter in accordance with the principles of the present invention;

FIG. 2 is an enlarged cross-sectional view taken along line 2-2 of FIG.1;

FIG. 3 is a top plan view of one of the hairpin shaped conductors of thehairpin resonator disposed between the input and output hairpinresonators illustrating the dimensions thereof;

FIG. 4 is a top plan view of a hairpin shaped conductor of the hairpinresonator that may be used in the input or output resonator of thefilter of FIG. 1 illustrat ing the dimensions thereof;

FIG. 5 is a curve of the unloaded Q as a function of the substratethickness at 7,500 MHz (megahertz) on a 99.5 percent alumina substratefor both shielded and unshielded hairpin resonators; and

FIG. 6 illustrates the frequency response of the nine pole filteremploying the shielded microstrip hairpin resonators of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1, 2, 3 and4, there is disclosed therein a microstrip filter operable at X-band inaccordance with the principles of the present invention. The microstripfilter includes N microstrip open circuited, half-wavelength hairpinresonators l-9 including a ground plane 10, a dielectric substrate 11composed of 99.5 percent alumina and N hairpin shaped conductors la-9adisposed in series coupled relationship with each other on one surfaceof substrate 11 opposite ground plane where N is equal to nine in theembodiment illustrated. It should be noted, however, that the microstripfilter in accordance with the principles of the present invention is notlimited to a microstrip filter having nine poles or resonator, but mayinclude any number of poles or resonators greater than two.

As illustrated in FIG. 3 each of the N conductors 2a-8a of hairpinresonators 2-8 include a pair of parallel spaced limbs 12 and 13.Resonators 2-8 are those resonators disposed between input resonator land output resonator 9. The spaced limbs 12 and 13 are interconnected bya cross-arm 14. It should be noted that adjacent ones of the conductorsla-9a have opposite orientation with respect to each other and adjacentones of the limbs of adjacent ones of the conductors la9a are in spacedparallel relationship with respect to each other to provide the seriescoupled relationship for the entire microstrip filter.

A microstrip input coupler 15 includes a coaxial connector l6 and aconductor 17 disposed on the same surface of the substance as conductorsla-9a. Conductor 17 includes a first portion 18 parallel to an adjacentlimb of conductor la and a second portion 19 extending perpendicular toportion 18 and connected to the inner conductor of the coaxial conductor16.

A microstrip output coupler 20 includes a coaxial connector 2] and aconductor 22 disposed on the same surface of substrate 11 as conductorsla-9a and includes a first portion 23 parallel to and spaced from theadjacent limb of conductor 9a and a second portion 24 disposedperpendicular to portion 23 connected to portion 23 and to the innerconductor of coaxial conductor 21.

Conductors 2a-8a have the dimensions illustrated in FIG. 3, namely, aconductor width W, a cross-arm length A, a height L and a spacingbetween limbs 12 and 13 equal to d wherein the value of W equals thevalue ofd and equals the value of h, the thickness of substrate FIG. 4illustrates the configuration of the input and output resonators la and90, respectively, wherein limb 13a adjacent portion 18 has a width W1,which is less than width W. The width W] of limb 13a is equal to thewidth of portion 18 with this width being carried through a section ofthe cross-arm 14a. The reduced width of limb 13a and cross-arm 14aincreases the impedance of the input conductor la. The other limb 12 ofconductor la adjacent conductor 2a has the same width W as the otherconductors 2a-8a. Portion l8 and a section of portion 19 of inputconductor 17 has the width W1. The remainder of portion 19 has its widthincreased to match the impedance of the input source coupled to coaxialline coupler 16. The same configuration is employed in conjunction withoutput conductor 9a and conductor 22 as described hereinabove withrespect to the input conductor la and conductor 17. It should be notedthat to compensate for the resultant shift in frequency due to thechange of impedance of the input and output conductor Ia and 9a ofresonators 1 and 9 that the dimension d (the spacing between the limbs)is increased to an amount equal to dl as illustrated in FIG. 4.

Metallic members 25 and 26 together with metallic cover 27 arecoextensive with ground plane 10 and substrate 11 and have anon-critical horizontal dimension x and a non-critical verticaldimension y to enclose the surface of substrate 11 containing conductorsla9a of resonators l-9 and the conductor portions of input conductor 17and output conductor 22 to shield the hairpin resonators in a manner toincrease the unloaded Q of the filter with respect to the unloaded Qobtainable with unshielded hairpin resonators.

FIG. 5 illustrates a pair of curves relating to the unloaded Q (Qu) as afunction of the thickness of substrate 10 at 7,500 MI-Iz on a 99.5percent alumina substrate 10. Curve 28 illustrates Qu for variousthickness h of substrate 10 for an unshielded hairpin resonator whilecurve 29 illustrates the Qu for various substrate thickness h forshieldedhairpin resonators in accordance with the principles of thepresent invention.

FIG. 6 illustrates the frequency response of the nine pole filteremploying shielded microstrip hairpin resonators of FIG. 1.

Referring to FIG. 2, it is illustrated therein that substrate 11 isseparated from ground plane 10 by an amount determined by the height ofmetal carrier 30 which is employed to test and adjust the variousresonators of the filter contained on substrate 11 outside of the finalassembly, the final assembly being illustrated in FIGS. 1 and 2. Theinclusion of the metal carrier or testing jig 30 in the final assemblymaintains the environment of the filter in the final assemblysubstantially identical with that present in the testing jig. Theemployment of the testing jig in the final assembly and the advantagethereof are fully described in the copending application of C. Greenwaldand M. Digrindakis, Ser. No. 191,284, filed Oct. 2l, 1971, now abandonedthe disclosure of said copending application being incorporated in thepresent application by reference.

The use of hairpin resonators of the open circuited, half-wavelengthresonator type reduces the resonation losses significantly at theexpense of a slight increase of resistive loss. The curves of FIG. 5summarize the rebecomes lower than upon some thinner substrates. If Qumust be higher than 310, shielded microstrip on substrates thicker than25 mil must be used.

In a reduction to practice the nine pole filter shown in FIG. 1 wasbuilt on a 50 mil alumina substrate and shielded by a channel having thedimension x 0.250 inches and the dimension y 0.300 inches. It should bepointed out, however, that the dimensions x and y are not critical tothe operation of the filter in accordance with the principles of thepresent invention. The hairpin resonator was used in the-embodiment ofFIG. 1 despite its percent lower Qu than the linear resonator because (1a more compact structure is obtained, one that can be placed in awaveguide of higher cutoff frequency, to ensure a predictable responseto about GI-Iz (gigahertz); (2) the metal shielding structure is lesscritical because the hairpin resonator already eliminates the greaterpart of the radiation; and (3) coupling between non-adjacent resonatorsis greatly reduced, which diminishes the ripple caused by such coupling.

One of the problems that was uncovered in testing the microstrip filteremploying the technique of the present invention is the difficulty ofcoupling into and out of the filter structure. This problem was solvedby increasing the impedance of the first and last resonator bydecreasing the width of the limb of the conductor thereof to have thesame width as a portion of the input and output coupling conductorscoupled to the coaxial connectors 16 and 21. The increase in impedanceachieved by changing the width of the limb of the input and outputresonators l and 9 to have an impedance equal to the input and outputconductors l7 and 22 resulted in a shift in frequency which wascorrected by a change in the spacing between the limbs of conductors laand 9a to a value of d1, as illustrated in FIG. 4, which is greater thanthe value d illustrated in FIG. 3 for resonators 28.

The curve illustrated in FIG. 6 is the measured frequency response ofthe filter of FIG. 1. The insertion loss of l.9db (decibels) correspondsto a Qu of 460 for a 0. 1 db Chebyshev design with a fractionablebandwidth of 7.4 percent. The out-of-band rejection was found to beexcellent up to 12.4 GHz, the limit of the measuring equipment. Belowthe passband, the rejection is greater than 75db while above thepassband the rejection was 65db.

A major objection to microstrip filters for use at X- band has been lowout-of-band rejection. This has been eliminated by the design andtechnique presented herein. The losses, another objection, can bereduced by a simple microstrip design as incorporated in the filtor ofthe present invention and FIG. 5 indicates that even lower losses arepossible. The necessity of enclosing the substrate in a metal channelfor shielding purposes is not a limitation since the filter must beshielded from other circuits if current RFI (radio frequency interference) specifications are to be met.

While I have described above the principles of my invention inconnection with specific apparatus it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention as set forth in the objects thereof and inthe accompanying claims.

I claim:

1. A microstrip filter comprising:

N microstrip open circuited, half-wavelength hairpin resonatorsincluding a ground plane, a substrate disposed adjacent to, coextensivewith and parallel to said ground plane and N hairpin shaped conductorsdisposed in series coupled relationship with each other on one surfaceof said substrate opposite said ground plane, where N is equal to aninteger greater than one, each of said N conductors including a pair ofparallel spaced limbs interconnected by a cross-arm, adjacent ones ofsaid N conductors having an opposite orientation with respect to eachother and adjacent ones of said limbs of adjacent ones of said Nconductors being in spaced parallel relationship with respect to eachother to provide said series coupled relationship;

a microstrip input coupler including a first conductor disposed on saidone surface of said substrate in spaced, parallel relationship with anadjacent one of said limbs of a first of said N conductors to provide aseries coupled relationship between said first conductor and saidadjacent one of said limbs of said first of said N conductors;

a microstrip output coupler including a second conductor disposed onsaid one surface of said substrate in spaced, parallel relationship withan adjacent one of said limbs of a last of said N conductors to providea series coupled relationship between said second conductor and saidadjacent one of said limbs of said last of said N conductors;

each of said N conductors and said first and second conductors being inalignment;

metallic members coextensive with said ground plane and said substrateenclosing said one surface of said substrate and said first, second andN conductors to shield said hairpin resonators to increase the unloadedQ of said resonators with respect to the unloaded Q obtainable withunshielded hairpin resonators.

2. A filter according to claim 1, wherein each of said limbs and saidcross-arm of each of said N conductors between said first and last ofsaid N conductors have a width equal to a first given amount;

the spacing between each of said limbs of each of said N conductorsbetween said first and last of said N conductors is equal to said firstgiven amount;

said first conductor includes a first portion parallel to said adjacentone of said limbs of said first of said N conductors and a secondportion perpendicular to said first portion, said first portion and asection of said second portion directly connected to said first portioneach have a width equal to a second given amount less than said firstgiven amount;

said second conductor includes a third portion parallel to said adjacentone of said limbs of said last of said N conductors and a fourth portionperpendicular to said third portion, said third portion and a section ofsaid fourth portion directly connected to said third portion each have awidth equal to said second given amount; and

said adjacent one of said limbs of each of said first and last of said Nconductors and a section of said cross-arms directly connected to saidadjacent one of said limbs of each of said first and last of said Nconductors have a width equal to said second given amount, the other ofsaid limbs of each of said first and last of said N conductors and theremainder of said cross-arms connected to said other of said iimbs ofeach of said first and last of said N conductors have a width equal tosaid first given amount and the spacing between said adjacent one andsaid other of said limbs of each of said first and last of said Nconductors is greater than said first given amount.

3. A filter according to claim 2, wherein the thickness of saidsubstrate is equal to said first given amount.

4. A filter according to claim 3, wherein said substrate is composed of99.5 percent alumina.

5. A filter according to claim I, wherein said substrate and said groundplane is separated by a metal carrier employed to test and adjust saidfilter prior to providing a final assembly thereof, said testing andadjusting taking place at a point removed from said final assembly.

6. A microstrip filter comprising:

N microstrip open circuited, half-wavelength hairpin resonatorsincluding a ground plane, a substrate disposed adjacent to, co-extensivewith and parallel to said ground plane and N hairpin shaped conductorsdisposed in series coupled relationship with each other on one surfaceof said substrate opposite said ground plane, where N is equal to aninteger greater than one, each of said N conductors including a pair ofparallel spaced limbs interconnected by a cross-arm, adjacent ones ofsaid N conductors having an opposite orientation with respect to eachother and adjacent ones of said limbs of adjacent ones of said Nconductors being in spaced parallel relationship with respect to eachother to provide said series coupled relationship;

a microstrip input coupler including a first conductor disposed on saidone surface of said substrate in spaced, parallel relationship with anadjacent one of said limbs of a first of said N conductors to provide aseries coupled relationship between said first conductor and saidadjacent one of said limbs of said first of said N conductors; and

a microstrip output coupler including a second conductor disposed onsaid one surface of said substrate in spaced, parallel relationship withan adjacent one of said limbs of a last of said N conductors to providea series coupled relationship between said second conductor and saidadjacent one of said limbs of said last of said N conductors;

each of said N conductors and said first and second conductors being inalignment;

each of said limbs and said cross-arm of each of said N conductorsbetween said first and last of said N conductors having a width equal toa first given amount;

the spacing between each of said limbs of each of said N conductorsbetween said first and last of said N conductors is equal to said firstgiven amount;

said first conductor including a first portion parallel to said adjacentone of said limbs of said first and said N conductors and a secondportion perpendicular to said first portion, said first portion and asection of said second portion directly connected to said first portioneach having a width equal to a second given amount less than said firstgiven amount;

said second conductor including a third portion parallel to saidadjacent one of said limbs of said last of said N conductors and afourth portion perpendicular to said third portion, said third portionand a section of said fourth portion directly connected to said thirdportion each having a width equal to said second given amount; and

said adjacent one of said limbs of each of said first and last of said Nconductors and a section of said cross-arms directly connected to saidadjacent one of said limbs of each of said first and last of said Nconductors having a width equal to said second given amount, the otherof said iimbs of each of said first and last of said N conductors andthe remainder of said cross-arms connected to said other of said limbsof each of said first and last of said N conductors having a width equalto said first given amount and the spacing between said adjacent one andsaid other of said limbs of each of said first and last of said Nconductors is greater than said first given amount.

7. A filter according to claim 6, wherein the thickness of saidsubstrate is equal to said first given amount.

8. A filter according to claim 7, wherein said substrate is composed of99.5 percent alumina.

9. A filter according to claim 8, wherein said substrate and said groundplane is separated by a metal carrier employed to test and adjust saidfilter prior to providing a final assembly thereof, said testing andadjusting taking place at a point removed from said final assembly.

10. A filter according to claim 6, wherein said substrate and saidground plane is separated by a metal carrier employed to test and adjustsaid filter prior to providing a final assembly thereof, said testingand adjusting taking place at a point removed from said final assembly.

1. A microstrip filter comprising: N microstrip open circuited,half-wavelength hairpin resonators including a ground plane, a substratedisposed adjacent to, coextensive with and parallel to said ground planeand N hairpin shaped conductors disposed in series coupled relationshipwith each other on one surface of said substrate opposite said groundplane, where N is equal to an integer greater than one, each of said Nconductors including a pair of parallel spaced limbs interconnected by across-arm, adjacent ones of said N conductors having an oppositeorientation with respect to each other and adjacent ones of said limbsof adjacent ones of said N conductors being in spaced parallelrelationship with respect to each other to provide said series coupledrelationship; a microstrip input coupler including a first conductordisposed on said one surface of said substrate in spaced, parallelrelationship with an adjacent one of said limbs of a first of said Nconductors to provide a series coupled relationship between said firstconductor and said adjacent one of said limbs of said first of said Nconductors; a microstrip output coupler including a second conductordisposed on said one surface of said substrate in spaced, parallelrelationship with an adjacent one of said limbs of a last of said Nconductors to provide a series coupled relationship between said secondconductor and said adjacent one of said limbs of said last of said Nconductors; each of said N conductors and said first and secondconductors being in alignment; metallic members coextensive with saidground plane and said substrate enclosing said one surface of saidsubstrate and said first, second and N conductors to shield said hairpinresonators to increase the unloaded Q of said resonators with respect tothe unloaded Q obtainable with unshielded hairpin resonators.
 2. Afilter according to claim 1, wherein each of said limbs and saidcross-arm of each of said N conductors between said first and last ofsaid N conductors have a width equal to a first given amount; thespacing between each of said limbs of each of said N conductors betweensaid first and last of said N conductors is equal to said first givenamount; said first conductor includes a first portion parallel to saidadjacent one of said limbs of said first of said N conductors and asecond portion perpendicular to said first portion, said first portionand a section of said second portion directly connected to said firstportion each have a width equal to a second given amount less than saidfirst given amount; said second conductor includes a third portionparallel to said adjacent one of said limbs of said last of said Nconductors and a fourth portion perpendicular to said third portion,said third portion and a section of said fourth portion directlyconnected to said third portion each have a width equal to said secondgiven amount; and said adjacent one of said limbs of each of said firstand last of said N conductors and a section of said cross-arms directlyconnected to said adjacent one of said limbs of each of said first andlast of said N conductors have a width equal to said second givenamount, the other of said limbs of each of said first and last of said Nconductors and the remainder of said cross-arms connected to said otherof said limbs of each of said first and last of said N conductors have awidth equal to said first given amount and the spacing between saidadjacent one and said other of said limbs of each of said first and lastof said N conductors is greater than said first given amount.
 3. Afilter according to claim 2, wherein the thickness of said substrate isequal to said first given amount.
 4. A filter according to claim 3,wherein said substrate is composed of 99.5 percent alumina.
 5. A filteraccording to claim 1, wherein said substrate and said ground plane isseparated by a metal carrier employed to test and adjust said filterprior to providing a final assembly thereof, said testing and adjustingtaking place at a point removed from said final assembly.
 6. Amicrostrip filter comprising: N microstrip open circuited,half-wavelength hairpin resonators including a ground plane, a substratedisposed adjacent to, co-extensive with and parallel to said groundplane and N hairpin shaped conductors disposed in series coupledrelationship with each other on one surface of said substrate oppositesaid ground plane, where N is equal to an integer greAter than one, eachof said N conductors including a pair of parallel spaced limbsinterconnected by a cross-arm, adjacent ones of said N conductors havingan opposite orientation with respect to each other and adjacent ones ofsaid limbs of adjacent ones of said N conductors being in spacedparallel relationship with respect to each other to provide said seriescoupled relationship; a microstrip input coupler including a firstconductor disposed on said one surface of said substrate in spaced,parallel relationship with an adjacent one of said limbs of a first ofsaid N conductors to provide a series coupled relationship between saidfirst conductor and said adjacent one of said limbs of said first ofsaid N conductors; and a microstrip output coupler including a secondconductor disposed on said one surface of said substrate in spaced,parallel relationship with an adjacent one of said limbs of a last ofsaid N conductors to provide a series coupled relationship between saidsecond conductor and said adjacent one of said limbs of said last ofsaid N conductors; each of said N conductors and said first and secondconductors being in alignment; each of said limbs and said cross-arm ofeach of said N conductors between said first and last of said Nconductors having a width equal to a first given amount; the spacingbetween each of said limbs of each of said N conductors between saidfirst and last of said N conductors is equal to said first given amount;said first conductor including a first portion parallel to said adjacentone of said limbs of said first and said N conductors and a secondportion perpendicular to said first portion, said first portion and asection of said second portion directly connected to said first portioneach having a width equal to a second given amount less than said firstgiven amount; said second conductor including a third portion parallelto said adjacent one of said limbs of said last of said N conductors anda fourth portion perpendicular to said third portion, said third portionand a section of said fourth portion directly connected to said thirdportion each having a width equal to said second given amount; and saidadjacent one of said limbs of each of said first and last of said Nconductors and a section of said cross-arms directly connected to saidadjacent one of said limbs of each of said first and last of said Nconductors having a width equal to said second given amount, the otherof said limbs of each of said first and last of said N conductors andthe remainder of said cross-arms connected to said other of said limbsof each of said first and last of said N conductors having a width equalto said first given amount and the spacing between said adjacent one andsaid other of said limbs of each of said first and last of said Nconductors is greater than said first given amount.
 7. A filteraccording to claim 6, wherein the thickness of said substrate is equalto said first given amount.
 8. A filter according to claim 7, whereinsaid substrate is composed of 99.5 percent alumina.
 9. A filteraccording to claim 8, wherein said substrate and said ground plane isseparated by a metal carrier employed to test and adjust said filterprior to providing a final assembly thereof, said testing and adjustingtaking place at a point removed from said final assembly.
 10. A filteraccording to claim 6, wherein said substrate and said ground plane isseparated by a metal carrier employed to test and adjust said filterprior to providing a final assembly thereof, said testing and adjustingtaking place at a point removed from said final assembly.